Dual Combustion Mode Engine

ABSTRACT

A system and method to provide a desired torque increase from an engine in which some cylinder operate in spark ignition (SI) combustion mode only and some cylinders operate in homogeneous-charge, compression-ignition (HCCI) combustion mode only are disclosed. Such method includes: operating a first portion of engine cylinders in homogeneous charge compression ignition combustion mode while a second portion of engine cylinders are deactivated. The second portion of engine cylinders in spark ignition combustion mode is activated when a torque demand is greater than the first portion of engine cylinders is capable of supplying. Also, the second portion of engine cylinders is activated when a rate of torque demand is greater than the first portion of engine cylinders is capable of supplying.

FIELD OF INVENTION

The present invention relates to an internal combustion engine in whicha portion of cylinders exclusively operate under a spark-ignitioncombustion mode and remaining cylinders exclusively operate under ahomogeneous-charge, compression-ignition mode.

BACKGROUND OF THE INVENTION

Homogeneous-charge, compression-ignition combustion is known to thoseskilled in the art to provide high fuel efficiency and low emissionoperation in internal combustion engines. However, HCCI operation isfeasible in a narrow range in engine torque, approximately one-third ofthe torque range of a conventional spark-ignited engine. Thus, most HCCIengines being developed are dual mode engines in which HCCI is used atlow torque conditions. When a higher torque is desired, operation istransitioned to an alternative combustion mode, such as spark-ignitioncombustion or heterogeneous, compression-ignition combustion (diesel).Challenges accompanying such transitions include: torque matching(providing driver demanded torque during the transition interval),maintaining emission control, and robustly returning to HCCI combustion,to name a few. Another difficulty encountered in engines whichtransition from one combustion mode to another is that the combustionsystem geometry cannot be optimized for either combustion mode, but isinstead a compromise. For example, a desirable compression ratio forHCCI combustion is about 15:1 and about 10.5:1 for spark-ignitioncombustion.

A disadvantage of HCCI combustion is its inferior transient response toa demand for a change in torque, orders of magnitude slower than SIcombustion. The inventors of the present invention have recognized thatHCCI operation cannot provide a vehicle operator with the responsivenessthat they have come to expect from a SI engine.

In U.S. patent application 2004/0182359, an 8-cylinder HCCI/SI engine isdescribed in which HCCI to SI transitions are made one cylinder at atime, i.e., at a lower torque demand all 8 cylinders are operating inHCCI combustion mode and as torque demand exceeds what HCCI combustioncan provide, cylinders are individually switched to SI operation. Theinventors of the present invention have recognized that it would bedesirable to have an engine which provides the desired range in outputtorque at the high efficiency of HCCI combustion without having toundergo a combustion mode transition in any given cylinder because ofthe compromises inherent in designing a cylinder to operate robustly andefficiently in both HCCI and SI combustion modes over a wide operatingrange.

SUMMARY OF THE INVENTION

A system and method to provide a desired torque increase from an enginein which some cylinder operate in SI combustion mode only and somecylinders operate in HCCI combustion mode only are disclosed. Suchmethod includes: operating a first portion of engine cylinders inhomogeneous charge compression ignition combustion mode while a secondportion of engine cylinders are deactivated. The second portion ofengine cylinders in spark ignition combustion mode is activated when atorque demand is greater than the first portion of engine cylinders iscapable of supplying. The second portion of engine cylinders isactivated when a rate of torque demand is greater than the first portionof engine cylinders is capable of supplying.

The first and second portions of cylinders are mutually exclusive, thefirst portion having a compression ratio at least two ratios greaterthan the second portion. When torque demand is reduced so that the firstportion of cylinder is capable of supplying the torque, the secondportion of engine cylinders is deactivated. Such cylinders aredeactivated by deactivating intake and exhaust valves coupled to thecylinders.

An advantage of the present invention is that the SI cylinders, whichhave a lesser fuel efficiency than HCCI cylinders, are utilized onlywhen needed to make up torque that the HCCI cylinders cannot provide.This makeup torque can be due to a torque level that the HCCI cylindersare incapable of providing or a rate of torque increase that the HCCIcylinders are incapable of providing. By relying on HCCI cylinderswhenever possible, fuel efficiency is optimized.

Also disclosed is a method to respond to an increased demand for torquein an engine in which a portion of engine cylinders are operating inHCCI mode and the remaining cylinders are operating in SI mode wherebyfuel delivery to the SI cylinders is increased to meet the demand. Thefuel supplied to the HCCI cylinders stays roughly constant. At this newtorque level, fuel delivery to the first portion of cylinders isincreased and fuel delivery to the second portion of cylinders isdecreasing, the increasing and decreasing such that demanded torquelevel is provided by the engine.

An advantage of the present invention is that it overcomes the inferiorresponse of HCCI engines by activating SI cylinders to provide thedesired torque increase. That is, it is not necessary to endure the slowramp up of HCCI cylinders to meet a torque increase demand. Instead, theSI cylinders are brought online immediately to fill a torque gap.Following such an activation of SI cylinders, torque from the HCCIcylinders is ramped up and if HCCI can provide the desired torque level,the SI cylinders are once again deactivated once the HCCI torque hasramped up. By using the HCCI cylinders as much as possible to providethe desired torque, the fuel efficiency is improved.

In the event that HCCI cylinders are incapable of providing the level oftorque desired, SI cylinders remain activated. However, torque isprovided by HCCI cylinders using SI cylinders as little as possible,i.e., just to provide makeup torque. Again, this provides the advantageof the best fuel efficiency at a particular engine operating condition.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment in which the invention is used to advantage,referred to herein as the Detailed Description, with reference to thedrawings wherein:

FIG. 1 is a schematic of an internal combustion engine according to anaspect of the present invention;

FIGS. 2 a-2 c are graphs of torque vs. fuel efficiency for prior art SIand HCCI engines and an engine according to an aspect of the presentinvention;

FIG. 3 is a flowchart of a cold start strategy;

FIGS. 4 a and 4 b are graphs of driver demanded torque over time for anengine according to the present invention; and

FIG. 5 is a schematic of the exhaust gas recirculation according to anaspect of the present invention.

DETAILED DESCRIPTION

In FIG. 1, a multi-cylinder internal combustion engine 10 is shown. Byway of example, engine 10 is shown to have six cylinders, two banks of 3cylinders each. One bank of cylinders 12 is adapted to operate in aconventional spark-ignition (SI) mode. The other bank of cylinders 14 isadapted to operate in a homogeneous-charge, compression-ignition (HCCI)mode. Combustion air is provided to the cylinder banks via an intakemanifold 30, which is separated such that air for cylinder bank 12 doesnot mix with air for cylinder bank 14. Each bank is provided with athrottle valve 24, or other means to control flow. SI combustioncharacteristically occurs at stoichiometric proportions of air and fuel,meaning that if combustion were complete, all the fuel and oxygen wouldbe completely combusted to H₂O and CO₂. To control the amount of torqueproduced in SI combustion, the amount of air is controlled by throttlevalve 24. The amount of fuel added to the air, via injector 26, ismetered to provide a stoichiometric mixture. For clarity, only one fuelinjector 26 is shown in FIG. 1 for the 3 cylinders of bank 12. However,each cylinder is provided a fuel injector. Similarly, each cylinder inbank 14 is provided with a fuel injector like fuel injector 28. In FIG.1, bank 12 is shown with a port injector in which fuel is sprayedoutside of the cylinder and is brought into the cylinder with thecombustion air and bank 14 is shown with a direct fuel injector in whichfuel is sprayed directly into the combustion air which has been inductedinto the cylinder. These types of fuel injection systems are shown byway of example. Both banks could be provided with port injection or bothwith direct injection according to the present invention. I.e., bothHCCI and SI combustion can be accomplished with port, direct, or acombination of port and direct injection. Alternatively, fuel isprovided by central body injection for either combustion mode.

Again, for clarity, only one of the three spark plugs for each of thecylinders of bank 12 is shown. Bank 14 cylinders may also have sparkplugs. Although the bank 14 cylinders are HCCI cylinders, whichindicates that combustion is initiated by compression ignition, it isknown to those skilled in the art that at some operating conditions, itis useful to employ spark assist to initiate combustion. Alternatively,another ignition assist device such as glow plugs, plasma jet igniters,catalytic assisted glow plugs, as examples, could be used for ignitionassist in HCCI. In SI combustion, a spark initiates a flame kernel and aflame front travels throughout the cylinder. In spark assisted HCCI, aspark initiates a flame kernel at the location of the spark plug.However, the mixture in the cylinder is too weak (not enough fuel or toomuch burned gases in the mixture) to sustain a flame front travelingthrough the cylinder gases. The flame kernel combusts the fuel-airmixture near the spark plug. The release of energy by the combustion ofthe mixture near the spark plug increases the pressure in the cylinderthereby causing the gases away from the spark plug to attain theirignition temperature and to self-ignite. When spark assist HCCI iscontemplated, all HCCI cylinders are provided with a spark plug 58.

Engine 10 is shown to be a 6-cylinder with bank 12 being SI and bank 14being HCCI by way of example. This is not intended to be limiting.Engine 10 has any number of cylinders greater than one and in anyconfiguration: in-line, V, W, radial, opposed, or any other cylinderarrangement. The HCCI and SI cylinders need not be segregated by banks.There could be HCCI and SI cylinders on any given bank. However, asmentioned above, the intake gases to the HCCI cylinders and SI cylindersremain separated and exhaust gases coming from HCCI cylinders and SIcylinders also remain separated. Thus, such arrangements may requirecomplicated manifolding to maintain the separation. An expectedarrangement is that every other cylinder in the firing order isalternately HCCI and SI.

SI engines are typically produced with a 9.5-10.5:1 compression ratio,which is the ratio of the volume in the cylinder above the piston whenthe piston is at the top of its travel divided by the volume in thecylinder above the piston when the piston is at the bottom of itstravel. HCCI combustion occurs more favorably with a higher compressionratio: 13-15:1. In prior art engines in which combustion mode istransitioned, the compression ratio selected is a compromise between thetwo compression ratios. According to the present invention, however,because each cylinder is optimized for a single combustion mode, theengine is produced with the compression ratio appropriate for theparticular combustion mode. Thus, unlike prior art engines, the engineaccording to the present invention has some cylinders with asubstantially higher compression ratio than other cylinders.

HCCI combustion occurs in a dilute mixture, either very lean ofstoichiometric with excess air and/or with a very high level of exhaustdilution. It is well known to those skilled in the art to provideexhaust dilution by either recirculating exhaust gases into the engineintake, known as exhaust gas recirculation (EGR) sometimes referred toas external EGR, or to retain exhaust gases in the cylinder from a priorcombustion event to mix with the combustion gases of an upcomingcombustion event, commonly known as internal EGR. The latter is oftenachieved by valve timing adjustments. Typically exhaust gases are routedfrom an exhaust duct to an intake duct via a control valve (EGR valve).The present invention provides for an alternative configuration for EGRin which gases exhausted from the SI cylinder bank 12 are routed to theintake of the HCCI cylinder bank 14 via valve 39. In FIG. 1, the exhaustgases are collected downstream of exhaust gas aftertreatment device 20.This is shown by way of example and not intended to be limiting. Theexhaust gases can be taken from any position in the exhaust duct. Thereare two advantages for circulating exhaust gases from the SI bank 12 tothe HCCI bank 14. Typically, SI combustion occurs with a stoichiometricmixture, which provides combustion gases containing primarily CO₂, H₂O,and N₂. In contrast, HCCI cylinders combust lean mixtures which haveexcess air. Thus, HCCI exhaust gas has significant levels of O₂ and moreN₂ than a SI exhaust gas. To obtain a desired diluent fraction, agreater amount of HCCI exhaust gas is recycled compared with the SIexhaust gas quantity. It is well known to those skilled in the art thatto achieve ignition in HCCI, it is common to heat the intake air.Because the exhaust gas temperature is higher with SI combustion, lessintake heating is required when the EGR employed comes from SIcombusting cylinders. In particular, with reference to FIG. 1, gas fromthe exhaust duct coupled to cylinder bank 12 is drawn through EGR system39 and supplied to the intake of cylinder bank 14 via a control valve.

Continuing with FIG. 1, each cylinder bank is provided with an exhaustgas aftertreatment device, 20 and 22. In one embodiment, device 20 is athree-way catalyst, which efficiently oxidizes CO and hydrocarbons andreduces nitrogen oxides (NOx) when provided a stoichiometric exhaustgas. As mentioned above, obtaining higher fuel efficiency motivates HCCIdevelopment. Another advantage of HCCI combustion, which occurs in avery lean or dilute mixture, is that it produces very low levels of NOx,particularly compared to SI operation. In one embodiment, the HCCIcylinders require no NOx aftertreatment and aftertreatment device 22 isan oxidation catalyst to process unburned fuel and CO. In anotherembodiment, a lean NOx aftertreatment device is employed to process thelow levels of NOx when very low NOx levels are required or when HCCIoperation is extended into regions at which the NOx produced is somewhathigher than typical HCCI combustion. In this embodiment, the lean NOxaftertreatment device is either a lean NOx trap or a lean NOx catalyst.A lean NOx trap stores NOx during lean operation. When the trap is nolonger able to store additional NOx, the trap is purged by operatinglean for a period of time. During the lean operation, the NOx isdesorbed from the trap and reacted to N₂ and O₂. To accomplish a richexcursion with HCCI, one alternative is to operate with a very highlevel of EGR to displace excess air. A lean NOx catalyst processes NOxin the presence of a reductant, either fuel or urea.

In FIG. 1, an indication of exhaust gas constituents is provided by anexhaust gas sensor, 60 and 62, situated in the exhaust ducts exitingeach cylinder bank. Only a single sensor is shown in FIG. 1. Exhaust gassensor 60 is an oxygen sensor, either heated or unheated, which providesan indication of whether the exhaust gas is near stoichiometry. Inanother embodiment, sensor 6o is a wide-range oxygen sensor provides ameasure of exhaust gas stoichiometry. Exhaust gas sensor 62 measures NOxconcentration. Alternatively, sensor 62 is a wide-range oxygen sensor.Only one exhaust gas sensor is shown in each of the exhaust ducts ofengine 10. However, it is known to use multiple exhaust gas sensors. Inone embodiment, both a wide-range oxygen sensor and a NOx sensor areplaced in place of sensor 62. Furthermore, it is common practice toprovide a sensor both upstream and downstream of an exhaustaftertreatment device. The inventors of the present inventioncontemplate any known exhaust gas sensor type in any location in theexhaust ducts.

The signal from an exhaust gas oxygen sensor 60 is commonly used forair-fuel ratio feedback control of SI combustion. Analogously, HCCIcombustion timing is controlled by adjusting intake temperature,according to one alternative embodiment. Adjustment of intaketemperature is feedback controlled based on a combustion parameter suchas crank angle of peak pressure. Examples of sensors from which crankangle of peak pressure can be ascertained include: head bolt straingauge, in-cylinder pressure sensor, ionization sensor, a head gasketsensor, sensor measuring instantaneous flywheel speed, etc. Forstoichiometric SI combustion, it is well known by those skilled in theart that the crank angle of peak pressure corresponding to peakefficiency operation (at a given speed/torque condition) occurs roughlyat 15 degrees after top dead center. Alternative combustion systems,particular lean burn, tend to have the crank angle of peak pressureoccur at a somewhat earlier time, e.g., 12 degrees after top dead centerto achieve peak efficiency. Furthermore, there are other objectives,besides achieving peak efficiency, such as emission control, which causethe desired crank angle of peak pressure to be other than that providingpeak efficiency. It is expected that a desired crank angle of peakpressure is in a range of 5 to 20 degrees after top center. Variouscombustion control parameters, such as: intake temperature, EGR valveposition, throttle valve position, flow through an intake heatexchanger, and pressure charging, can be feedback controlled based oncrank angle of peak pressure, particularly for the HCCI cylinders.

Because HCCI combustion is dilute, the peak torque capable from a givencylinder is much less than peak torque from a SI cylinder. To increasethe amount of torque from a HCCI cylinder, compressor 34 increases theintake manifold pressure on the HCCI cylinders, allowing for anincreased amount of fuel delivery while maintaining a high dilution. Asshown in FIG. 1, compressor 34 is connected by a shaft to turbine 32, adevice known as a turbocharger. The unconventional ducting of FIG. 1 hasturbine 32 extracting work from SI cylinder exhaust gases which compressintake gases of HCCI cylinders via compressor 34. HCCI combustion isknown to provide superior fuel efficiency to SI combustion. Thus, HCCIexhaust gases have a lower enthalpy than SI exhaust gases because HCCIallows more of the energy release of combustion to be extracted. Thus,it is desirable to extract the SI exhaust gas energy to pressure chargethe HCCI inlet. In another embodiment, exhaust turbine 32 is coupled tothe HCCI exhaust duct. The turbocharger of FIG. 1 is a variable geometryturbocharger. In yet another embodiment, a supercharger is provided inplace of turbocharger (comprising elements 32, 34, and 36). Asupercharger is a compressor, like compressor 34 of FIG. 1, which isdriven by engine 10. A supercharger is not coupled to a turbine.

In FIG. 1, an intake gas heat exchanger 38 is contained with the exhaustduct coupled to the SI cylinders. It is known in the art that one of themethods to control ignition timing of HCCI combustion is by controllingthe intake temperature. Diverter valve 37 allows adjustment of thequantity of HCCI intake gases passing through heat exchanger 38 and thequantity passing through bypass duct 35, thereby providing control ofHCCI intake temperature.

Continuing to refer to FIG. 1, electronic control unit (ECU) 40 isprovided to control engine 10. ECU 40 has a microprocessor 46, called acentral processing unit (CPU), in communication with memory managementunit (MMU) 48. MMU 48 controls the movement of data among the variouscomputer readable storage media and communicates data to and from CPU46. The computer readable storage media preferably include volatile andnonvolatile storage in read-only memory (ROM) 50, random-access memory(RAM) 54, and keep-alive memory (KAM) 52, for example. KAM 52 may beused to store various operating variables while CPU 46 is powered down.The computer-readable storage media may be implemented using any of anumber of known memory devices such as PROMs (programmable read-onlymemory), EPROMs (electrically PROM), EEPROMs (electrically erasablePROM), flash memory, or any other electric, magnetic, optical, orcombination memory devices capable of storing data, some of whichrepresent executable instructions, used by CPU 46 in controlling theengine or vehicle into which the engine is mounted. Thecomputer-readable storage media may also include floppy disks, CD-ROMs,hard disks, and the like. CPU 46 communicates with various sensors andactuators via an input/output (I/O) interface 44. Examples of items thatare actuated under control by CPU 46, through I/O interface 44, arecommands to fuel injectors 26 and 28 such as fuel injection timing, fuelinjection rate, and fuel injection duration. Additional parameters underCPU 46 control are position of throttle valves 24, timing of spark plug58, position of control EGR valves 39, other control valves 37, variablegeometry turbocharger nozzle position, intake and exhaust valve timing,and others. Sensors 42 communicating input through I/O interface 44 mayinclude piston position, engine rotational speed, vehicle speed, coolanttemperature, intake manifold pressure, accelerator pedal position,throttle valve position, air temperature, exhaust temperature, exhauststoichiometry, exhaust component concentration, and air flow. Some ECU40 architectures do not contain MMU 48. If no MMU 48 is employed, CPU 46manages data and connects directly to ROM 50, RAM 54, and KAM 52. Ofcourse, the present invention could utilize more than one CPU 46 toprovide engine control and ECU 40 may contain multiple ROM 50, RAM 54,and KAM 52 coupled to MMU 48 or CPU 46 depending upon the particularapplication.

Referring to FIG. 2 a, a graph of thermal efficiency as a function oftorque is shown for a conventional SI engine as curve 100. Curve 104indicates the higher thermal efficiency that is possible when operatingthe same displacement engine in the HCCI combustion mode. The thermalefficiency is markedly improved. However, HCCI does not provide the samerange in torque as a SI engine of the same displacement. To provide thesame torque level, either the displacement of the engine has to beroughly doubled or combustion mode is changed from HCCI at low torqueand then SI when higher torque is demanded.

Also shown in FIG. 2 a are curves 102 and 106, which is only half of theengine cylinders operating on SI and HCCI respectively. The peak thermalefficiency is the same as the corresponding curves 100 and 104 when theengine is operated with all cylinders on SI and HCCI, respectively. Therange in torque, is half that of the running the whole engine.

According to an aspect of the present invention, half of the cylindersare operated with HCCI combustion and half of the cylinders are operatedwith SI combustion, the effect of such operation on torque and thermalefficiency being shown in FIG. 2 b. Note that FIG. 2 b relates to anaturally aspirated engine in which there is no supercharger orturbocharger to pressurize intake gases. Because of the high efficiencyof HCCI combustion, it is desirable to operate only the HCCI cylindersat low torque demands. Thus, curve 106 of FIGS. 2 a and 2 b areidentical, i.e., half of the cylinders operating on HCCI and the othercylinders being deactivated. When a higher demand in torque is desired,SI cylinders are activated and torque is being provided by both the SIand HCCI cylinders, shown as curve 108 in FIG. 2 b. Because HCCIcylinders cannot provide the same torque range as SI, the peak torque inFIG. 2 b is less than what is shown in FIG. 2 a. The efficiency shown inFIG. 2 b exceeds that of the SI engine efficiency, curve 102, of FIG. 2a at all values of torque.

To make up for the lesser torque of the engine illustrated in FIG. 2 b,either the displacement of the engine can be increased or boostingapplied. Boosting can be applied to the HCCI cylinders or all enginecylinders. However, as discussed above, since HCCI provides less torque,an approach for HCCI cylinders to match torque is to boost only HCCIcylinders. Curve 116 of FIG. 2 c shows thermal efficiency where boost isprovided by a supercharger. The torque range of curve 116 is wider thanthat of curve 106 (of FIG. 2 b) due to the increased amount of airdelivered to the HCCI cylinders by the supercharger. The torque level atwhich half HCCI and half SI (curve 118 of FIG. 2 c) is invoked it higherthan in FIG. 2 b. The maximum torque in FIG. 2 c is about that of FIG. 2a.

If a turbocharger were employed in place of a supercharger, no increasein torque range with HCCI only operation is possible because the SIcylinders are deactivated, thus no exhaust to drive the turbocharger.

Because achieving a sufficiently high temperature to cause autoignitionis paramount in HCCI combustion, providing a robust cold start presentsa serious hurdle for HCCI combustion. Those skilled in the art discussstarting on SI combustion and transitioning to HCCI combustion after theengine has achieved a suitable operating temperature. However, with thepresent invention, the cylinders are adapted to operate in only onecombustion mode. To overcome, the inventors of the present inventioncontemplate starting on SI cylinders. During the period of SIcombustion, air can be delivered to HCCI cylinders through heatexchanger 38. By blowing warm air through the HCCI cylinder bank 14, theengine surfaces can be preheated and ready for HCCI combustion. Inaddition, the engine coolant is heated by the SI cylinders and preheatsthe HCCI cylinders.

According to an aspect of the present invention, the SI cylinders areequipped with valve deactivators (not shown). During HCCI onlyoperation, the SI cylinders are deactivated by closing off the intakeand exhaust valves. The piston continues to reciprocate, but the gas inthe cylinder at the last combustion event remains trapped in thecylinder. If the valves were allowed to remain active, the flow of airthrough SI cylinder bank 12 would flow into exhaust aftertreatmentdevice 20. If device 20 is a three-way catalyst, oxygen would beabsorbed onto the surfaces and when the SI cylinders were reactivated,the three-way catalyst would be unable to reduce NOx until such oxygenis removed from device 20. Furthermore, the flow of air through SIcylinder bank 20 cools the engine down, thereby making restart moredifficult.

In one embodiment, valve deactivators are provided for the HCCIcylinders (not shown in FIG. 1). However, because HCCI combustion ismore efficient than SI combustion, it is desirable to operate the HCCIcylinders whenever possible. When the HCCI cylinders have not attained asuitable operating temperature, the HCCI cylinders can be deactivated.However, according to an aspect of the invention discussed above,exhaust waste energy from SI cylinders is transferred to HCCI inductionair via heat exchanger 38. Thus, it may be preferable to allow valves inHCCI cylinders to operate as normal to allow heating.

Referring to FIG. 3, a cold start is initiated in block 150. The enginestarts operation on SI cylinders only, block 152. Induction air flowsthrough heat exchanger 38 and then through HCCI cylinders to warm up theHCCI cylinders, block 154. In block 156, it is determined whether asufficient temperature has been achieved. If so, operation in the HCCIcylinders is initiated in block 158. If the torque that can be producedby the HCCI cylinders is sufficient to meet torque demand (determined inblock 160), control is passed to block 162 in which the SI cylinders aredeactivated. If not enough torque can be produced in the HCCI cylinders,the engine is operated with all cylinders active, block 164.

In FIG. 4 a, a hypothetical driver torque demand as a function of timeis shown as line 200. Between time o and time a, the driver demands arelatively low torque, at time a, the driver pushes down the acceleratorpedal, which indicates demand for higher torque. The driver againdemands increased torque at times b and c. Between times o and a, onlythe HCCI cylinders are active. Since the HCCI combustion mode providessuperior fuel economy and the HCCI cylinders are capable of providingthe desired torque, the SI cylinders are deactivated. As the torquedemand isn't changing, there is no concern about the slow transientresponse of HCCI combustion. When the transition to a higher torque isdemanded at time a, the HCCI cylinders cannot respond quickly enough toattain the new, higher torque. At time a′, the HCCI cylinders haveattained the desired torque level. However, between time a and a′, ifonly HCCI cylinders were active, the torque response of the vehiclewould be unacceptable to the operator of the vehicle. Thus, according tothe present invention, the SI cylinders are activated at time a so thatthe operator demanded torque can be more closely followed than with HCCIcylinders alone. As the HCCI cylinders ramp up their torque production,the SI cylinders are ramped down and eventually deactivated at time a′.At time b, another rapid torque increase is demanded. Again, the SIcylinders are activated to fill in and the torque produced by the HCCIcylinders is ramped up. As at time a when a rapid torque increase isdemanded, the HCCI cylinders are too slow to provide the transientresponse requested. Thus, the SI cylinders are reactivated at time b. Attime b′, the HCCI engines have attained their maximum torque outputcondition. Thus at time b′, the SI cylinders remain active. At time c,the further increase in demanded torque is provided solely by the SIcylinders because the HCCI cylinders are already operating at their peakcapacity.

In FIG. 4 b, a similar increase in torque is shown, however with a muchslower demand for transient torque response. From time o to time b′, thedriver demand is slow enough that the HCCI cylinders can provide thedesired torque demand. At time b′, the HCCI cylinders have reached theircapacity and the SI cylinders are activated. Further increases in torqueare provided by the SI cylinders.

In another embodiment, the SI cylinders remain active at all times. Inone example of this embodiment, an 8-cylinder is started on 2 SIcylinders. The remaining 6 cylinders are HCCI cylinders, which areturned on when they reach a suitable temperature which supports robustHCCI combustion. In this embodiment, the SI cylinders remain operationaleven after HCCI combustion has been achieved in the 6 HCCI cylinders.

Referring now to FIG. 5, a HCCI bank 14 and a SI bank 12 areincorporated in engine 10. The arrangement of FIG. 5 is provided simplyfor ease of illustration. As discussed above, various arrangements arecontemplated by the inventors of the present invention, in one exampleeach bank has one HCCI cylinder and three SI cylinders with complicatedmanifolding to keep their intake and exhaust gases separated.) Bank 12is provided with exhaust 16 and bank 14 is provided with exhaust 18. Aportion of exhaust gases from bank 12 may be drawn off by an EGR systemthrough EGR valve 70. Similarly, exhaust gases are drawn from an exhaustfrom bank 14 through EGR valve 72. Both EGR ducts flow into the intaketo bank 14. The intake to bank 14 is also supplied with throttle valve24. Throttle valve 24, EGR valve 70 and EGR valve 72 are controlled byelectronic control unit 40.

Because HCCI combustion is very dilute, HCCI combustion gases are at amuch lower temperature than SI combustion gases. By controlling theproportion of EGR gases coming from bank 12 and from bank 14, thetemperature in HCCI cylinders is controlled. As mentioned above, one ofthe ways, known by those skilled in the art, for controlling HCCIcombustion timing is by adjusting the temperature of the gases in theHCCI cylinder. By continuing to operate SI cylinders while HCCIcylinders are operating, the exhaust gases from SI cylinders isavailable for recycle to the HCCI cylinders for controlling temperaturein HCCI cylinders.

While several modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize alternative designs and embodiments for practicing theinvention. The above-described embodiments are intended to beillustrative of the invention, which may be modified within the scope ofthe following claims.

1. A method for operating an internal combustion engine, comprising:operating a first portion of engine cylinders in homogeneous chargecompression ignition combustion mode wherein a second portion of enginecylinders are deactivated; and activating said second portion of enginecylinders in spark ignition combustion mode when a torque demand isgreater than said first portion of engine cylinders is capable ofsupplying.
 2. The method of claim 1 wherein a fueling rate to said firstportion of engine cylinders is decreased when said second portion ofcylinders is activated, said first and second portions comprise allengine cylinders, and said first portion and said second portion aremutually exclusive.
 3. The method of claim 1, further comprising:deactivating said second portion of engine cylinders when said torquedemand is lessened such that said first portion of engine cylinders iscapable of supplying said torque.
 4. The method of claim 1 wherein acompression ratio of said first portion of engine cylinders is more thantwo ratios less than a compression ratio of said second portion ofengine cylinders.
 5. An internal combustion engine, comprising: a firstportion of engine cylinders having a compression ratio less than 11.5:1;a second portion of engine cylinders having a compression ratio greaterthan 13.5:1; and an electronic control unit coupled to said first andsecond portions of cylinders, said electronic control unit deactivatingsaid second portion of engine cylinders when said first portion ofengine cylinders is capable of supplying demanded torque wherein saidsecond portion of engine cylinders operates in a homogeneous chargecompression ignition combustion mode.
 6. (canceled)
 7. The engine ofclaim 5 wherein said electronic control unit activates said secondportion of engine cylinders when said first portion of engine cylindersprovides less torque than demanded.
 8. The engine of claim 7 whereinsaid second portion of engine cylinders operates in a spark ignitioncombustion mode.
 9. The engine of claim 7 wherein said activationincludes activating intake and exhaust valves coupled to cylinders ofsaid first portion of engine cylinders, activating fuel injectorscoupled to cylinders of said first portion of engine cylinders, andproviding signals to fire spark plugs coupled to cylinders of said firstportion of engine cylinders.
 10. A method of operating an internalcombustion engine, comprising: operating a first portion of enginecylinders in a homogeneous charge compression ignition combustion mode;operating a second portion of engine cylinders in a spark ignitioncombustion mode; and increasing fuel delivery to said second portion ofcylinders and maintaining fuel delivery substantially constant to saidfirst portion of cylinders in response to a torque increase demand. 11.The method of claim 10 wherein said first portion of cylinders have acompression ratio at least two ratios greater than said second portionof cylinders.
 12. (canceled)
 13. The method of claim 10, furthercomprising: increasing fuel delivery to said first portion of cylinders;and decreasing fuel delivery to said second portion of cylinders whereinsaid increasing fuel to said first portion of cylinders and saiddecreasing to said second portion of cylinders are accomplished aftersaid increasing of fuel delivery to said second portion of cylinders andin such a manner that said increased torque demand is provided by theengine.
 14. A method for operating an internal combustion engine,comprising: operating a first portion of engine cylinders in homogeneouscharge compression ignition combustion mode wherein a second portion ofengine cylinders are deactivated; and activating said second portion ofengine cylinders in spark ignition combustion mode when a rate of torquedemand is greater than said first portion of engine cylinders is capableof supplying.
 15. The method of claim 14, further comprising: increasingtorque supplied by said first portion of engine cylinders and decreasingtorque supplied by said second portion of engine cylinders in responseto said torque demand.
 16. The method of claim 14, further comprising:deactivating said second portion of engine cylinders when said firstportion of engine cylinders is capable of providing said demandedtorque.
 17. The method of claim 16 wherein said deactivating comprisesdeactivating intake and exhaust valves coupled to said second portion ofengine cylinders such that said intake and exhaust valves remain closed.18. The method of claim 17 wherein said deactivating further comprisesdiscontinuing fuel supply to said second portion of engine cylinders.19. A method of operating an internal combustion engine, comprising:operating a first portion of engine cylinders in a homogeneous chargecompression ignition combustion mode; activating a second portion ofengine cylinders in a spark ignition combustion mode in response to ademand for an increased torque.
 20. The method of claim 19 wherein saidfirst portion of engine cylinders is capable of providing said demandedtorque increase but with an unacceptable torque response.
 21. The methodof claim 19 wherein after said demanded torque increase is satisfied byactivating said second portion of engine cylinder, the method furthercomprising: reducing an amount of fuel supplied to said second portionof engine cylinders; and increasing an amount of fuel supplied to saidfirst portion of engine cylinders, such that said demanded torqueincrease is maintained through said reducing and increasing.
 22. Themethod of claim 21, further comprising: deactivating said second portionof engine cylinders when said demanded torque increase can be suppliedby said first portion of engine cylinders.